1. Field of the Invention
The present invention is directed to the preparation of poly(ethylene glycol) and poly(ethylene oxide) by initiated chemical vapor deposition.
2. Description of the Related Technology
Chemical vapor deposition (CVD) of inorganic thin films is a widely used technology for the fabrication of integrated circuits in the semiconductor industry. A specialized version of CVD, initiated chemical vapor deposition (iCVD), is used for polymeric thin film synthesis. iCVD typically uses lower temperatures compared to CVD thus allowing synthesis of linear polymers without undesirable side groups or cross-linking and essentially perfect retention of functional groups in the polymer. iCVD polymerization has been demonstrated via the free radical polymerization mechanism for acrylates, methacrylates, styrenic monomers, and other vinyl monomers.
Chemical vapor deposition (CVD) is a broad term that includes thermal CVD that uses high substrate temperatures, e.g. in silicon dioxide growth; plasma enhanced CVD (PECVD) that uses high electromagnetic (DC, RF, microwave) excitation energy, e.g. in plasma polymerization; hot filament CVD (HFCVD) that uses high filament temperature e.g. in diamond and amorphous silicon; and iCVD that uses much lower substrate and filament temperatures than HFCVD enabling polymer deposition and coating of temperature sensitive surfaces. PECVD suffers from the additional disadvantage that it excessively breaks down the precursor leading to cross-linking and loss of chemical functionality during the polymerization process.
Poly(ethylene glycol) (PEG) is a technologically important polymer with many biomedical applications including tissue engineering, drug delivery, non-biofouling membranes and spatial patterning of cells. Ethylene oxide was used as the monomer in conjunction with anionic as well as cationic initiators. PEG is referred to as poly(ethylene oxide) (PEO) when its molecular weight is higher than 20,000 g/mol.
Conventional methods for the preparation of poly(ethylene glycol) and poly(ethylene oxide) rely on liquid phase synthesis and processing. As such, these methods employ solvents and liquids and thus suffer from solvent constraints, solvent toxicity and contamination problems, solvent-induced morphology changes in the polymer, liquid surface tension that prevents conformal coating of micro and nanostructures, and thick coatings. iCVD overcomes these challenges by bypassing the liquid phase, enabling control over film thickness down to the nanoscale, conformally coating and shrinkwrapping the substrate, including complex 3D architectures, and enabling the tunability of polymer chemistries/properties not possible with solvent-constrained systems.
An all-dry encapsulation method that enables well-defined polymers to be applied around particles of sizes down to the nanoscale is known from, for example, WO 2007/145657A2. In certain embodiments, the encapsulation method is a modified form of initiated chemical vapor deposition (iCVD) using a thermally-initiated free radical polymerization to create conformal coatings around individual particles while avoiding agglomeration. In another embodiment, iCVD may be used to encapsulate fine drug microcrystals (e.g., below 100 μm in size) with methacrylic acid copolymers (such as poly(methacrylic acid-co-ethyl acrylate) and poly(methacrylic acid-co-ethylene dimethyacrylate)) for the purpose of conferring enteric release properties. iCVD is said to provide a uniform or substantially uniform coating on rough, fibrous, and porous morphologies with high surface areas. By using controlled radical polymerization chemistries, iCVD produces exceptionally clean polymers with stoichiometric compositions, high molecular weights and having no residual solvents, excipients, glidants or plasticizers. With plasma-based dry methods, fully functional linear polymers are not produced because the high-energy plasma environment results in non-selective chemistries which lead to crosslinked networks.
The initiators used in free radical polymerization are molecules that yield radicals by bond dissociation for initiating chain polymerization, such as peroxides where the oxygen-oxygen bond is weak, or azo compounds where the nitrogen-nitrogen bonds is weak. In making free radical polymers, typically the monomers contain carbon-carbon double bonds susceptible to the free radical in order to propagate the polymer chain.
Also, the reaction conditions used for free radical polymerization may be different from the reaction conditions required for other types of polymerization such as ionic polymerization or ring-opening polymerization. As such, it would be very difficult for one skilled in free radical polymerization to transfer that knowledge to ionic polymerization.
Accordingly, there is a need in the art for polymerization and coating processes which employ ionic polymerization methods in order to provide additional desirable products including polymers which cannot be typically made by free radical initiated polymerization reactions. These and other objects and aspects of the invention will be apparent from the summary and detailed descriptions which follow.